Method of applying pigmented coating to sheet metal



United States Patent METHOD OF APPLYING PIGMENTED COATING TO SHEET METAL Thomas L. Cannilf, Wheaton, Ill., assignor to American 'Can Company, New York, N. Y., a corporation of New Jersey No Drawing. Application February 16, 1955 Serial No. 488,717

3 Claims. (Cl. 117-71) The present invention relates generally to a method of coating a web of sheet metal with a pigmented varnish. More particularly the invention pertains to the method of applying an aluminum pigmented varnish to sheet steel to form thereon a bright, highly reflective finish.

Sheet steel or black iron as it is sometimes called is generally quite susceptible to corrosion. To prevent such corrosion it is the general practice to cover the exposed surface of the black iron with a protective coating. Since the surface of black iron is relatively dull, the selected coating in addition to its corrosion protective properties should also enhance the appearance of the black iron surface by providing it with a brilliant and reflective surface, as in the case of tin plate.

In recent years efforts have been made to conserve the use of tin by finding eifective substitutes therefor. One of these substitutes has been the use of finely divided aluminum dispersed in a vehicle comprising a film forming resin and a solvent. Heretofore black iron surfaces have been coated with aluminum by applying the aluminum pigment-resin-solvent composition to the surfaces and then evaporating the solvent as by heat or other means so that the aluminum pigment is held on the surface of the substrate metal by means of the synthetic resin film. Although this method of coating black iron with aluminum produces a film which is satisfactory as a protective coating for the substrate metal, the indiscriminate arrangement of the aluminum particles in the film gives the film a lack-lustre, grainy appearance which is not pleasing to the eye.

It is therefore an objectof the present invention to provide a method of coating which improves the appearance of dull surfaced sheet metal.

It is another object of the invention to provide a method of giving dull surfaced sheet metal a bright, lustrous finish.

A further object of the invention is to provide a method of coating black iron with a corrosion resistant film containing aluminum.

Yet another object of the present invention is to provide a method whereby black iron may be quickly, efficiently and economically given an attractive, protective coating containing aluminum.

Numerous other objects and advantages of the invention will be apparent as it is better understood from the following description which is of a preferred embodiment thereof.

I have discovered that the above and other objects may be achieved by applying to the black iron or other dull surfaced metal a first coating of film forming resin dissolved in a suitable solvent; then applying over the first coating While it is still wet, a second coating comprising a film forming resin compatible with or at least adherent to the film forming resin of the first coating, a suitable solvent therefor, and finely divided aluminum dispersed throughout this vehicle; and then evaporating the solvent from both coatings in a single operation as by heat to cause the resin thereinto solidify and harden. I have discovered that by means of this procedure the aluminum particles or pigment are caused to stratify and concentrate near'the surface of the combined coatings to give a smooth, lustrous, light reflective finish to the coating. Also the resins in the two layers of wet coating can be set and cured in a single operation thereby obviating the extra handling and time required in separately setting and curing the resins in each coating. An additional advantage obtained by the instant invention is that because a certain amount of intermingling or flowing together occurs in the vicinity of the interface of the two fluid coatings, upon hardening, the top coat becomes firmly adhered or locked to the base coat.

The resins useful in the present invention have a dual function of forming a protective film over the substrate metal and also bonding the aluminum to the substrate. To carry out the first function, the resin must be capable of forming a tough continuous film which adheres tightly to the substrate metal. Further since these resins are to be applied in solution it is necessary that they be soluble in a volatile solvent. The resins must also be capable of being cured by the application of heat.

Any of the well known resins both natural and synthetic having the above desired properties are operable in the present invention. Examples of such resins are alkyd resins, i. e. those formed from the condensation polymerization of a polybasic acid and a polyhydric alcohol, such as phthalic, maleic or succinic acid with glycerine or glycols; non-heat reactive phenolic resins such as those prepared from 2,2-bis-(4-hydroxyphenyl) propane, i. e. bis-phenol and epichlorhydrin and other resins of this genus described in United States Patent 2,503,726 to S. O. Greenl'ee; oleoresins; synthetic drying oils such as liquid copolymers of styrene and butadiene as described in United States Patent 2,652,342 to A. H. Gleason; melamine resins; and urea-formaldehyde resins.

Since it is necessary that the aluminum containing top coat be firmly anchored to the subjacent or base coat, it is preferred that the resins in each of these two coats be the same or at least chemically similar. By this means the possibility of separating or peeling the top coat from the base coat is completely obviated. However, if desired a different resin may be used in each coat providing the resins are compatible and/or adherent to one another. For example, an alkyd resin top coat may be applied over an oleoresinous base coat.

Because the aluminum containing top coat is to be applied to the base coat while the latter is still wet, i. e. still containing solvent, it is necessary that the solvents in each coat be compatible, i. e. either the same or at least miscible with each other. Failure to use such compatible solvents results in the repulsion of the top coat by the base coat whereby the top coat forms into isolated droplets or curds. Also, solvents which have a tendency to react with the aluminum pigment, such as cellulose ethers, must be avoided. Aromatic and aliphatic hydrocarbon solvents such as xylene and mineral spirits, either alone or in admixture with small amounts of ketones, e. g. methyl ethyl ketone, or alcohols, e. g. butanol, are useful in the present invention.

The aluminum pigment used in the present invention is known as leafing aluminum. This type of pigment is in the form of very small, thin, flat plates. When applied according to the method of the present invention, this leafing pigment orients itself to dispose the flat surface of each leaflet in substantial parallelism with the surface of its fluid vehicle and in overlapping relation with one another, thereby resulting in a uniformly continuous metallic lustre finish.

To afford protection for the metal sheet, it is necessary that suflicient solids, principally resin, be deposited on the sheet. As will be more fully explained hereinafter the quantity of resin in the top coat must be maintained at a relatively low level. Therefore the predominant amount of resin needed for the protection of the metal sheet must be contained in the base coat. Also, the less solvent that must be removed from the base coat during hardening thereof, the more eflicient the process. This finding is based on the fact that the solvent that is removed from the wet base coat must pass through the top coat. It has been observed that if too great an amount of solvent must be removed in this manner, cracking and checking of the top coat may occur. Finished coatings in which the weight of the base coat is from 2 to 4 times the weight of the top coat have been found to give excellent protection to the metal sheet and at the same time present an appearance that has a bright, metallic lustre.

For the rapid and efiicient application of a base coat and top coat of the desired weights, the base coat should be applied with as high a viscosity as possible and the application viscosity of the top coat should be as low as possible. Generally speaking, the viscosity of the top coat should be less than half the viscosity of the base coat.

The minimum operable viscosity of the base coat is about 50 seconds. The viscosities disclosed herein are all measured at 80 F. using a #4 Ford cup. The maximum viscosity limitation is controlled only by the fact that the coating must be in solution during application for ease and simplicity of operation and by the ability of applying means to handle the high viscosity material. In actual operation, the preferred viscosity is from 60 to 80 seconds.

For the top pigment carrying coat it is preferred that the dispersion of metal pigment in film forming resin solution be diluted with additional solvent to a point where the application viscosity of the top coat is substantially equal to the viscosity of the solvent, i. e. in the order of to seconds. When applied, this low viscosity of the top coat forms a thin continuous liquid film over the surface of the wet base coat, and allows the leaflets of aluminum pigment to orient themselves readily to form a continuous lustrous, light reflective layer near the surface of the top coat. Also, in making the viscosity of the top coat as low as possible, the resistance of the top coat to the passage of solvent vapors therethrough from the underlying wet base coat is minimized so as to avoid disturbance or damaging of the top coat.

To provide this lustrous finish it is desirable to have approximately the same weight of the metal pigment as film forming resin in the top coat. If too great an excess of resin over aluminum is present in the top coat, proper orientation of the metal pigment does not occur and too great a thickness of film forming resin is disposed over the surface of the metal pigment leaflets whereby the lustre and light reflective quality of the metal pigment is seriously impaired if not destroyed. On the other hand, if the metal pigment is in too great an excess, there is insuflicient resin to bond it to the coating whereby the pigment is easily smudged or flaked off.

Solvent removal from the coatings and the curing of the resins therein may be carried out. under normal temperatures such as room temperature or at elevated temperature. For the sake of time economy it is preferred to carry out the hardening of the resin at elevated temperatures. The optimum elevated temperature will depend upon the particular resin used in each coating. However it can be stated generally that the temperature will be in excess of 100 F. and below the temperature at which the resin will decompose or become charred and preferably about from 325 F. to 410 F. For temperatures within this range times of from 6 to 10 minutes will suflice to set and cure the resin.

The apparatus used for theapplication of the top coat to the fluid, i. e. -wet,-base coat, must be such as to avoid disturbance of the base coat. If the base coat is disturbed to an excessive degree, the finished coating will exhibit disfiguring lines or flow marks. Although it is possible to spray the successive coatings onto the metal sheet by careful control of the variables involved, e. g. air pressure, distance of the spray nozzle from the work, etc.,"l prefer to apply the base coat to the metal sheet and the top coat thereover by means of roller coating, i. e. the coatings are transferred to the sheet from a cylinder rotating in a bath of coating solution. By-this means, the roller applying the top coating can be readily adjusted so that it provides just enough pressure to deposit the top coating over the wet base coat without producing undesirable disturbance or excessive flow marks in the base coat.

The following example is by way of illustration only and is not to be construed as limiting the invention:

Example Into the reservoir of a roller coating machine is placed a solution having a viscosity of 75 :5 seconds of an alkyd resin dissolved in a mixture of Solvesso 100 (hydrocarbon having an aromatic content of to 99.5% and a distillation range of 306 F. to 343 F.) and mineral spirits. Into a second roller coating machine adjacent to and in tandem with the first roller coating machine is placed a slurry composed of leafing aluminum pigment dispersed in a solution of an alkyd resin and equal parts of Solvesso and mineral spirits. This slurry contains equal parts of aluminum pigment and alkyd resin film-former and has a viscosity of 19 to 20 seconds. Black iron sheets are fed through the first coater where a film of'alkyd resin in solution is applied to yield a coating weight when measured in a semi-wet condition of 10 to 12 mgs. per four square inches of metal sheet. Immediately after the application of the first coating (elapsed time 2 to 4 seconds), the black iron sheets containing the wet first coating are passed through the second coating machine where a film of the aluminum pigment vehicle is deposited on the wet first coating in an amount sufficient to yield a coating weight of 3 to 5 mgs. per four square inches of surface as measured in a semi-wet condition. The thus coated sheets are then baked at a temperature of 400 F. for 10 minutes during which time the solvent in both base and top coatings is evaporated and each coating cures to a hard, tough film. The black iron sheets having the finished coating thereon have a bright, metallic lustre, free of graininess or a mottled effect.

If desired, the lustre and brightness of the coated sheet may be further enhanced by bufling. This result is possible because by the method of the present invention the metal particles are concentrated near the surface of the film where they are available to the bufling and polishing action. Also, any scratches produced on the surface of the coating during handling of the coated plate can be made less noticeable by the incorporation of a small percentage of metal pigment in the base coating vehicle.

Sheet metal coated in the manner of the present invention is well suited for the fabrication of container parts. The bright, lustrous, silvery appearance of thin gauge sheet steel or black iron coated by the instant method make such metal plate an excellent substitute for tin plate in the manufacture of metal containers both from the standpoint of corrosion resistance and appearance.

Although the present invention has been described in relation to aluminum pigmented coating on black iron, the subject invention is also applicable to the coating of other types of web material, for example paper, and when using coatings containing other pigments, such as different metals, e. g. copper, and a wide variety of inorganic pigments.

It is thought that the invention and many of its attendant advantages Will be understood from the foregoing description, and it will be apparent that various changes may be made in the steps of the method described and their order of accomplishment without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the method hereinbefore described being merely a preferred embodiment thereof.

I claim:

1. The method of coating a metal sheet to protect the same and to produce a lustrous finish thereon, comprising applying a fluid base coating to said sheet and thereafter applying a fluid top coating while said base coating is wet, said coatings containing compatible film-forming resins and compatible solvents to form adherent unbroken coating layers, said top coating further containing a pigment comprising metallic leaflets, said leaflets and said top coat resin having substantially equal parts by weight, said base coat having a weight per unit area and a viscosity each at least twice that of said top coat to produce a tough, protective coating on said sheet and to permit said base coat solvent to evaporate through said less viscous top coat without disturbing said pigment therein, whereby to produce a lustrous grainless finish on said sheet, and evaporating said solvents and curing said resins.

2. The method of coating a metal sheet to protect the same and to produce a lustrous finish thereon, comprising applying a fluid base coating to said sheet and thereafter applying a fluid top coating while said base coating is wet, said coatings containing compatible film-forming resins and compatible solvents to form adherent unbroken coating layers, said top coating further containing a leafing aluminum pigment, said pigment and said top coat resin having substantially equal parts by weight, said base coat having a weight per unit area of two to four times the weight of the top coat to produce a tough protective coating on said sheet, and having a viscosity of at least twice that of said top coat to permit said base coat solvent to evaporate through said less viscous top coat without disturbing said pigment therein, whereby to produce a lustrous grainless aluminum finish on said sheet, and evaporating said solvents and curing said resins.

3. The method of coating black iron sheet material to protect the same and to produce a lustrous grainless metallic finish comprising applying a base coating containing a film-forming resin dissolved in a volatile solvent and having a viscosity of -80 seconds at F. in a #4 Ford cup, applying a top coat over said base coat while said base coat is wet, said top coat containing a film-forming resin compatible with said base coat resin, a solvent compatible with said base coat solvent, and leafing aluminum pigment, said pigment and said top coat resin having substantially equal parts by weight, said top coat having a viscosity of less than onehalf that of said base coat to permit evaporation of said base coat solvent through said top coat without damaging or dislocating said aluminum pigment therein, and said base coat having a weight per unit area of at least twice that of said top coat to provide a tough protective film adjacent said sheet material, and baking said coated sheet material within a temperature range of 325 to 410 F. to evaporate said solvents and cure said resins, whereby said sheet material is provided with a hard tough protective film having a lustrous grainless aluminum finish.

References Cited in the file of this patent UNITED STATES PATENTS 602,797 Annison Apr. 19, 1898 1,521,055 Tesse Dec. 30, 1924 2,054,389 Rosenthal et al. Sept. 15, 1936 2,276,685 Bright Mar. 17, 1942 2,325,408 Kauppi et al July 27, 1943 2,568,004 Hcyman Sept. 18, 1951 OTHER REFERENCES Burns et a1.: Protective Coatings for Metals, Reinhold Publ. Co., N. Y., 1939. 

1. THE METHOD OF COATING A METAL SHEET TO PROTECT THE SAME AND TO PRODUCE A LUSTROUS FINISH THEREON, COMPRISING APPLYING A FLUID BASE COATING TO SAID SHEET AND THEREAFTER APPLYING A FLUID TOP COATING WHILE SAID BASE COATING IS WET, SAID COATINGS CONTAINING COMPATIBLE FILM-FORMING RESINS AND COMPATIBLE SOLVENTS TO FORM ADHERENT UNBROKEN COATING LAYERS, SAID TOP COATING FURTHER CONTAINING A PIGMENT COMPRISING METALLIC LEAFLETS, SAID LEAFLETS AND SAID TOP COAT RESIN SUBSTANTIALLY EQUAL PARTS BY WEIGHT, SAID BASE COAT HAVING A WEIGHT PER UNIT AREA AND A VISCOSITY EACH AT LEAST TWICE THAT OF SAID TOP COAT TO PRODUCE A TOUGH, PROTECTIVE COATING ON SAID SHEET AND TO PERMIT SAID BASE COAT SOLVENT TO EVAPORATE THROUGH SAID LESS VISCOUS TOP COAT WITHOUT DISTURBING SAID PIGMENT THEREIN, WHEREBY TO PRODUCE A LUSTROUS GRAINLESS FINISH ON SAID SHEET, AND EVAPORATING SAID SOLVENTS AND CURING SAID RESINS. 